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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 5 — Mar. 11, 2013
  • pp: 6519–6525
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Resonance enhancement of terahertz metamaterials by liquid crystals/indium tin oxide interfaces

Zhen Liu, Chia-Yi Huang, Hongwei Liu, Xinhai Zhang, and Chengkuo Lee  »View Author Affiliations


Optics Express, Vol. 21, Issue 5, pp. 6519-6525 (2013)
http://dx.doi.org/10.1364/OE.21.006519


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Abstract

This work fabricates a terahertz (THz) metamaterial device, whose structure consists of split ring resonator array/ plastic substrate/ Indium Tin Oxide (ITO) film/ liquid crystals/ ITO film/ plastic substrate. Experiment results show that the resonance of the THz metamaterial device can be enhanced as voltage is applied to the liquid crystals. The enhancement will be more significant as higher voltage applied. The resonance enhancement is attributed to the fact that the liquid crystals/ITO interfaces exhibit the large difference in terms of refractive index between the two materials in THz regime. The interfaces reflect the incident electromagnetic wave and cause the reflected wave to enhance the resonance of the metamaterials. As those frequency-tunable metamaterial devices show different resonant transmittance at different frequencies, which is undesired, the liquid crystals/ITO interfaces can improve those frequency-tunable metamaterial devices with a constant transmittance at different frequencies.

© 2013 OSA

1. Introduction

Because of their negative index property in the terahertz (THz) frequency regime, THz metamaterials has attracted great attention recently [1

1. H. T. Chen, J. F. O'Hara, A. K. Azad, A. J. Taylor, R. D. Averitt, D. B. Shrekenhamer, and W. J. Padilla, “Experimental demonstration of frequency-agile terahertz metamaterials,” Nat. Photonics 2(5), 295–298 (2008). [CrossRef]

5

5. D. Shrekenhamer, W. C. Chen, and W. J. Padilla, “Liquid crystal tunable metamaterial perfect absorber,” arXib:1206.4214v1 (2012).

], while frequency tunability and board-band capability of THz metamaterials remain as technical challenges. Liquid crystals have received considerable interest in the development of tunable THz metamaterial devices due to the large birefringence of liquid crystals [5

5. D. Shrekenhamer, W. C. Chen, and W. J. Padilla, “Liquid crystal tunable metamaterial perfect absorber,” arXib:1206.4214v1 (2012).

8

8. F. Zhang, Q. Zhao, W. Zhang, J. Sun, J. Zhou, and D. Lippens, “Voltage tunable short wire-pair type of metamaterial infiltrated by nematic liquid crystal,” Appl. Phys. Lett. 97(13), 134103 (2010). [CrossRef] [PubMed]

]. In these studies, liquid crystals are used to act as the surrounding media of the metamaterials, as electrical field, magnetic field or heat is applied to the metamaterial devices embedded with liquid crystals, the resonance frequency of the metamaterials is shifted because the probe electromagnetic wave is experienced the variation in the permittivity or the permeability of the liquid crystals. As a drawback, when resonance frequency shifting, the magnitude of resonance transmittance is changed as well which is undesirable for commercial tunable electro-optic devices. It is of great interests that researchers want to develop an approach to a constant resonance intensity of the THz metamaterials even the resonance frequency of the metamaterial devices are tuned by the aforementioned methods.

This work fabricates a THz metamaterial device, where split ring resonator (SRR) array is deposited on a liquid crystal cell which is formed by two inward-facing indium tin oxide (ITO)-coated plastic substrates, to study the effect of the liquid crystals/ITO interfaces on the resonance of the SRR array. The reason for choosing ITO is due to its good conductivity so that we can bias liquid crystals by applying voltage and its large refractive index in THz regime. The liquid crystals are used because their refractive index can be controlled by varying the applied voltage to it. Experiment results show that the resonance of the SRR array is enhanced as a voltage is applied to the liquid crystal cell, and that an increase in the voltage makes the resonant transmittance lower. The resonance enhancement results from the coupling of the surface-plasmon-polarition-like mode and the electromagnetic wave reflected from the liquid crystals/ITO interfaces. The liquid crystals/ITO interfaces can be used to keep the resonant transmittance of the proposed metamaterial devices constant by external voltages [5

5. D. Shrekenhamer, W. C. Chen, and W. J. Padilla, “Liquid crystal tunable metamaterial perfect absorber,” arXib:1206.4214v1 (2012).

8

8. F. Zhang, Q. Zhao, W. Zhang, J. Sun, J. Zhou, and D. Lippens, “Voltage tunable short wire-pair type of metamaterial infiltrated by nematic liquid crystal,” Appl. Phys. Lett. 97(13), 134103 (2010). [CrossRef] [PubMed]

].

2. Preparation of samples

Figure 1(a)
Fig. 1 (a) Configuration of metamaterial device. (b) Dimensions of SRR unit: T = 6 μm, G = 20 μm, W = 50 μm and L = 60 μm. (c) Optical microscope image for the SRR array. (d) Photograph of the metamaterial device, which is pasted on a transparent cylindrical plastic bottle.
depicts the configuration of the THz metamaterial device. An empty cell is fabricated using two ITO-coated polyester (PET) substrates, which are separated by two 100-μm-thick spacers. The thicknesses (permittivities) of the ITO films and PET substrates, dITO and dPET (εITO and εPET), are 100 nm and 175 μm (6400 [9

9. S. A. Jewell, E. Hendry, T. H. Isaac, and J. R. Sambles, “Tuneable Fabry–Perot etalon for terahertz radiation,” New J. Phys. 10(3), 033012 (2008). [CrossRef]

] and 3.0 [10

10. Y. S. Jin, G. J. Kim, and S. G. Jeon, “Terahertz dielectric properties of polymers,” J. Korean Phys. Soc. 49(2), 513–517 (2006).

]), respectively. On the top PET substrate of the cell is deposited a 200-nm-thick aluminum SRR array using lift-off process. The empty cell is filled with E7 liquid crystals (Merck, ε=ne2=2.90and ε=no2=2.48 at THz [11

11. C. L. Pan and R. P. Pan, “Characterization and applications of liquid crystals in the THz frequency range,” Proc. SPIE 8279, 82790I (2012). [CrossRef]

]), then sealed with polymer gel. In order to achieve free polarization for the liquid crystals, no aligning layer has been adopted in the case. The reason of preferring liquid crystals with free polarization is to make the characterization simpler since no polarized light is needed as well. Figure 1(b) shows the dimensions of the SRR, where the line-width (T), gap (G), width (W) and length (L) are 6, 20, 50 and 60 μm, respectively. A reflective optical microscopy image of the SRR array is shown in Fig. 1(c). Both of the periods along x and y directions are 100 μm. As shown in Fig. 1(d), the flexibility and transparency of the metamaterial device are demonstrated by pasting it on a cylindrical plastic bottle.

This study will use external voltages to modify the permittivity of the liquid crystals, εLC. Owing to the orientation of the liquid crystals in the THz metamaterial device is isotropic, εLC at zero applied voltage is given by εiso=(ε+2ε)/3=2.62for a non-polarized THz electromagnetic wave which is normally incident to the metamaterial device. When the applied voltage is high enough, εLC will equal ε = 2.48 since the electrical field of the electromagnetic wave makes an angle of 90° with respect to the director. Therefore, the permittivity of the liquid crystals decreases with the increasing applied voltage.

3. Results and discussion

To study the effect of the liquid crystals/ITO interfaces on the resonance enhancement of the SRR array, a simulation, in which the configuration of a metamaterial device is the same as that in Fig. 1(a) while the device has no ITO films, is carried out by commercial finite-integration time-domain software. The material parameters that are used in the simulation refer to Section 2. Under εLC = εiso = 2.62 (i.e., at zero voltage), the resonant transmittance is lower in the metamaterial device with ITO films than the case without ITO films, as shown in Fig. 3(a)
Fig. 3 (a) Simulated transmission spectrums of metamaterial devices with and without ITO films under εLC = εiso = 2.62 (i.e., at zero voltage). (b) Simulated transmission spectrums of metamaterial device with ITO films with various εLC. (c) Simulated transmission spectrums of metamaterial device with a high-permittivity material (εM = 104).
. This result reveals that the ITO films of the metamaterial device play a key role in the resonance enhancement of the SRR array. Figure 3(b) shows the simulated transmittance spectrums of the metamaterial device with the ITO films under various εLC. These spectrums are used to study the responses of the metamaterial device at various voltages. In Fig. 3(b), the resonant transmittance decreases with the decreasing εLC (i.e., the increasing voltage). This simulated result is consistent with the experimental result of Fig. 2, and demonstrates that high difference between εLC and εITO results in the enhancement of the resonance of the SRR array. Moreover, as a material whose refractive index is higher than ITO in THz regime is substituted for the ITO films of Fig. 1(a), the resonant transmittance of the metamaterial device with the high-permittivity material (εM = 104) decreases in comparison with that of the metamaterial device with the ITO films, as shown in Fig. 3(c). The simulated results in Figs. 3(a), 3(b) and 3(c) confirm that the resonance enhancement of the metamaterial device with the ITO films results from the liquid crystals/ITO interfaces. Therefore, the difference of refractive index at the interfaces has the capability to modify the resonant transmittance of a THz metamaterial device.

In Fig. 2, the resonant frequency of the metamaterial device does not shift under the applied voltage. This is due to the fact that the thickness of the top PET substrate, dPET = 175 μm, is greater than its critical thickness, dc [13

13. Z. Schlesinger and A. J. Sievers, “IR surface-plasmon attenuation coefficients for Ge-coated Ag and Au metals,” Phys. Rev. B 26(12), 6444–6454 (1982). [CrossRef]

,14

14. J. Han, X. Lu, and W. Zhang, “Terahertz transmission in subwavelength holes of asymmetric metal-dielectric interfaces: the effect of a dielectric layer,” J. Appl. Phys. 103(3), 033108 (2008). [CrossRef]

]. dc can be written as
dc=c4fεPET1,
(2)
where c is the speed of the surface-plasmon-polarition-like mode in vacuum, and f refers to the resonant frequency. Substituting f = 0.62 THz and εPET = 3.0 into Eq. (2) yields dc = 81.6 μm. Since the condition dPET > dc is satisfied, the electric field of the surface-plasmon-polarition-like mode almost confines in the top PET substrate. Therefore, the resonant frequency of the metamaterial device depends mainly on the permittivity of the top PET substrate and remains constant under the applied voltages.

4. Application

The resonant transmittance, for the metamaterial device which adopts the liquid crystals/ITO interfaces to enhance the resonance, decreases with decreasing εLC, as shown in Fig. 3(b). Meanwhile, the resonant transmittance, for the metamaterial device which utilizes the liquid crystals to tunable the resonant frequency, increases with decreasing εLC, as shown in Fig. 4(b). The two results may be helpful to develop a frequency-tunable metamaterial device with a constant transmittance at different frequencies. Consider a metamaterial device, as shown Fig. 5(a)
Fig. 5 (a) Schematic drawing of stacked metamaterial device. (b) Simulated transmission spectrums of the device with various permittivities of the liquid crystals. εLC_top and εLC_bot are the permittivities of the top and bottom liquid crystal layers, respectively.
, which is formed by stacking the two metamaterial devices in Figs. 1(a) and 4(a) and then substituting a double-side ITO-coated PET substrate for the two single-side ITO-coated PET substrates. Figure 5(b) shows the simulated transmittance spectrums of the m for the top and bottom liquid crystal layers with different permittivities. In Fig. 5(b), the four resonant spectrums exhibit the same resonant transmittance. It is attributed the fact that the liquid crystals/ITO interfaces adjacent to the bottom liquid crystal layer makes the resonant transmittance of the SRR array constant. The liquid crystals/ITO interfaces has the ability to develop a frequency-tunable metamaterial device with a constant transmittance at different frequencies. The liquid crystals/ITO interfaces can be stacked on absorbers, cloaking and filters due to the flexible PET substrates, hence exhibiting its wide range of applications. Moreover, the liquid crystals/ITO interfaces have features such as easy fabrication and low cost. For future study, photoalignment which has the capability of modifying the permittivity of liquid crystals could be used in such metamaterial structures so as to achieve optically-controllable metamaterial devices [18

18. V. G. Chigrinov, V. M. Kozenkov, and H. S. Kwok, Photoalignment of Liquid Crystalline Materials: Physics and Applications (Wiley, 2008).

].

5. Conclusions

In conclusion, we report the THz metamaterial device whose resonant transmittance decreases with increasing voltages while its resonant frequency remains at the same value. The decrease in the resonant transmittance results from the liquid crystals/ITO interfaces with large difference in refractive index between these two materials in THz regime. The fixed resonant frequency is attributed to the fact that the electric field of the surface-plasmon-polarition-like mode almost confines in the top PET substrate and therefore the frequency depends mainly on the permittivity of the top PET substrate. By leveraging the switchable resonant transmittance and the fixed resonant frequency, we propose the improved method for those frequency-tunable metamaterial devices so that they can exhibit constant transmittance at different frequencies. Owing to that no aligning layer is needed in the THz metamaterial device, this device has a feature of free polarization for using in band-stop filters.

Acknowledgments

This work was supported in part by SERC of the Agency for Science, Technology and Research (A*STAR) Grant 1021650084 (Novel Sensors and Acoustic Transducer for the Down-hole Electronic System) at the National University of Singapore under Grants R-263000643305, and the Academic Research Committee Fund MOE2009-T2-2-011 (Nanoneedle devices for transdermal vaccine delivery) at the National University of Singapore under Grants R-263000598112.

References and links

1.

H. T. Chen, J. F. O'Hara, A. K. Azad, A. J. Taylor, R. D. Averitt, D. B. Shrekenhamer, and W. J. Padilla, “Experimental demonstration of frequency-agile terahertz metamaterials,” Nat. Photonics 2(5), 295–298 (2008). [CrossRef]

2.

W. J. Padilla, M. T. Aronsson, C. Highstrete, M. Lee, A. J. Taylor, and R. D. Averitt, “Electrically resonant terahertz metamaterials: theoretical and experimental investigations,” Phys. Rev. B 75(4), 041102 (2007). [CrossRef]

3.

H. Tao, C. M. Bingham, D. Pilon, K. Fan, A. C. Strikwerda, D. Shrekenhamer, W. J. Padilla, X. Zhang, and R. D. Averitt, “A dual band terahertz metamaterial absorber,” J. Phys. D Appl. Phys. 43(22), 225102 (2010). [CrossRef]

4.

N. R. Han, Z. C. Chen, C. S. Lim, B. Ng, and M. H. Hong, “Broadband multi-layer terahertz metamaterials fabrication and characterization on flexible substrates,” Opt. Express 19(8), 6990–6998 (2011). [CrossRef] [PubMed]

5.

D. Shrekenhamer, W. C. Chen, and W. J. Padilla, “Liquid crystal tunable metamaterial perfect absorber,” arXib:1206.4214v1 (2012).

6.

F. Zhang, W. Zhang, Q. Zhao, J. Sun, K. Qiu, J. Zhou, and D. Lippens, “Electrically controllable fishnet metamaterial based on nematic liquid crystal,” Opt. Express 19(2), 1563–1568 (2011). [CrossRef] [PubMed]

7.

Q. Zhao, L. Kang, B. Du, B. Li, J. Zhou, H. Tang, X. Liang, and B. Zhang, “Electrically tunable negative permeability metamaterials based on nematic liquid crystals,” Appl. Phys. Lett. 90(1), 011112 (2007). [CrossRef]

8.

F. Zhang, Q. Zhao, W. Zhang, J. Sun, J. Zhou, and D. Lippens, “Voltage tunable short wire-pair type of metamaterial infiltrated by nematic liquid crystal,” Appl. Phys. Lett. 97(13), 134103 (2010). [CrossRef] [PubMed]

9.

S. A. Jewell, E. Hendry, T. H. Isaac, and J. R. Sambles, “Tuneable Fabry–Perot etalon for terahertz radiation,” New J. Phys. 10(3), 033012 (2008). [CrossRef]

10.

Y. S. Jin, G. J. Kim, and S. G. Jeon, “Terahertz dielectric properties of polymers,” J. Korean Phys. Soc. 49(2), 513–517 (2006).

11.

C. L. Pan and R. P. Pan, “Characterization and applications of liquid crystals in the THz frequency range,” Proc. SPIE 8279, 82790I (2012). [CrossRef]

12.

E. Hecht, Optics (Addison Wesley, 2002), Chap. 4.

13.

Z. Schlesinger and A. J. Sievers, “IR surface-plasmon attenuation coefficients for Ge-coated Ag and Au metals,” Phys. Rev. B 26(12), 6444–6454 (1982). [CrossRef]

14.

J. Han, X. Lu, and W. Zhang, “Terahertz transmission in subwavelength holes of asymmetric metal-dielectric interfaces: the effect of a dielectric layer,” J. Appl. Phys. 103(3), 033108 (2008). [CrossRef]

15.

K. Y. Lo, C. Y. Huang, T. H. Chu, C. J. Hsu, C. H. Lin, and A. Y. G. Fuh, “Variation of nematic liquid crystal on a silver surface,” J. Opt. A, Pure Appl. Opt. 8(6), 501–506 (2006).

16.

A. Boltasseva and V. M. Shalaev, “Fabrication of optical negative-index metamaterials: recent advances and outlook,” Metamaterials (Amst.) 2(1), 1–17 (2008). [CrossRef]

17.

J. F. O’Hara, R. Singh, I. Brener, E. Smirnova, J. Han, A. J. Taylor, and W. Zhang, “Thin-film sensing with planar terahertz metamaterials: sensitivity and limitations,” Opt. Express 16(3), 1786–1795 (2008). [CrossRef] [PubMed]

18.

V. G. Chigrinov, V. M. Kozenkov, and H. S. Kwok, Photoalignment of Liquid Crystalline Materials: Physics and Applications (Wiley, 2008).

19.

F. Zhang, L. Kang, Q. Zhao, J. Zhou, X. Zhao, and D. Lippens, “Magnetically tunable left handed metamaterials by liquid crystal orientation,” Opt. Express 17(6), 4360–4366 (2009). [CrossRef] [PubMed]

20.

X. Wang, D. H. Kwon, D. H. Werner, I. C. Khoo, A. V. Kildishev, and V. M. Shalaev, “Tunable optical negative-index metamaterials employing anisotropic liquid crystals,” Appl. Phys. Lett. 91(14), 143122 (2007). [CrossRef]

21.

B. Zhu, Y. Feng, J. Zhao, C. Huang, and T. Jiang, “Switchable metamaterial reflector/absorber for different polarized electromagnetic waves,” Appl. Phys. Lett. 97(5), 051906 (2010). [CrossRef]

OCIS Codes
(160.3710) Materials : Liquid crystals
(160.3918) Materials : Metamaterials
(300.6495) Spectroscopy : Spectroscopy, teraherz
(230.7408) Optical devices : Wavelength filtering devices

ToC Category:
Metamaterials

History
Original Manuscript: November 6, 2012
Revised Manuscript: January 30, 2013
Manuscript Accepted: February 20, 2013
Published: March 8, 2013

Citation
Zhen Liu, Chia-Yi Huang, Hongwei Liu, Xinhai Zhang, and Chengkuo Lee, "Resonance enhancement of terahertz metamaterials by liquid crystals/indium tin oxide interfaces," Opt. Express 21, 6519-6525 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-5-6519


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References

  1. H. T. Chen, J. F. O'Hara, A. K. Azad, A. J. Taylor, R. D. Averitt, D. B. Shrekenhamer, and W. J. Padilla, “Experimental demonstration of frequency-agile terahertz metamaterials,” Nat. Photonics2(5), 295–298 (2008). [CrossRef]
  2. W. J. Padilla, M. T. Aronsson, C. Highstrete, M. Lee, A. J. Taylor, and R. D. Averitt, “Electrically resonant terahertz metamaterials: theoretical and experimental investigations,” Phys. Rev. B75(4), 041102 (2007). [CrossRef]
  3. H. Tao, C. M. Bingham, D. Pilon, K. Fan, A. C. Strikwerda, D. Shrekenhamer, W. J. Padilla, X. Zhang, and R. D. Averitt, “A dual band terahertz metamaterial absorber,” J. Phys. D Appl. Phys.43(22), 225102 (2010). [CrossRef]
  4. N. R. Han, Z. C. Chen, C. S. Lim, B. Ng, and M. H. Hong, “Broadband multi-layer terahertz metamaterials fabrication and characterization on flexible substrates,” Opt. Express19(8), 6990–6998 (2011). [CrossRef] [PubMed]
  5. D. Shrekenhamer, W. C. Chen, and W. J. Padilla, “Liquid crystal tunable metamaterial perfect absorber,” arXib:1206.4214v1 (2012).
  6. F. Zhang, W. Zhang, Q. Zhao, J. Sun, K. Qiu, J. Zhou, and D. Lippens, “Electrically controllable fishnet metamaterial based on nematic liquid crystal,” Opt. Express19(2), 1563–1568 (2011). [CrossRef] [PubMed]
  7. Q. Zhao, L. Kang, B. Du, B. Li, J. Zhou, H. Tang, X. Liang, and B. Zhang, “Electrically tunable negative permeability metamaterials based on nematic liquid crystals,” Appl. Phys. Lett.90(1), 011112 (2007). [CrossRef]
  8. F. Zhang, Q. Zhao, W. Zhang, J. Sun, J. Zhou, and D. Lippens, “Voltage tunable short wire-pair type of metamaterial infiltrated by nematic liquid crystal,” Appl. Phys. Lett.97(13), 134103 (2010). [CrossRef] [PubMed]
  9. S. A. Jewell, E. Hendry, T. H. Isaac, and J. R. Sambles, “Tuneable Fabry–Perot etalon for terahertz radiation,” New J. Phys.10(3), 033012 (2008). [CrossRef]
  10. Y. S. Jin, G. J. Kim, and S. G. Jeon, “Terahertz dielectric properties of polymers,” J. Korean Phys. Soc.49(2), 513–517 (2006).
  11. C. L. Pan and R. P. Pan, “Characterization and applications of liquid crystals in the THz frequency range,” Proc. SPIE8279, 82790I (2012). [CrossRef]
  12. E. Hecht, Optics (Addison Wesley, 2002), Chap. 4.
  13. Z. Schlesinger and A. J. Sievers, “IR surface-plasmon attenuation coefficients for Ge-coated Ag and Au metals,” Phys. Rev. B26(12), 6444–6454 (1982). [CrossRef]
  14. J. Han, X. Lu, and W. Zhang, “Terahertz transmission in subwavelength holes of asymmetric metal-dielectric interfaces: the effect of a dielectric layer,” J. Appl. Phys.103(3), 033108 (2008). [CrossRef]
  15. K. Y. Lo, C. Y. Huang, T. H. Chu, C. J. Hsu, C. H. Lin, and A. Y. G. Fuh, “Variation of nematic liquid crystal on a silver surface,” J. Opt. A, Pure Appl. Opt.8(6), 501–506 (2006).
  16. A. Boltasseva and V. M. Shalaev, “Fabrication of optical negative-index metamaterials: recent advances and outlook,” Metamaterials (Amst.)2(1), 1–17 (2008). [CrossRef]
  17. J. F. O’Hara, R. Singh, I. Brener, E. Smirnova, J. Han, A. J. Taylor, and W. Zhang, “Thin-film sensing with planar terahertz metamaterials: sensitivity and limitations,” Opt. Express16(3), 1786–1795 (2008). [CrossRef] [PubMed]
  18. V. G. Chigrinov, V. M. Kozenkov, and H. S. Kwok, Photoalignment of Liquid Crystalline Materials: Physics and Applications (Wiley, 2008).
  19. F. Zhang, L. Kang, Q. Zhao, J. Zhou, X. Zhao, and D. Lippens, “Magnetically tunable left handed metamaterials by liquid crystal orientation,” Opt. Express17(6), 4360–4366 (2009). [CrossRef] [PubMed]
  20. X. Wang, D. H. Kwon, D. H. Werner, I. C. Khoo, A. V. Kildishev, and V. M. Shalaev, “Tunable optical negative-index metamaterials employing anisotropic liquid crystals,” Appl. Phys. Lett.91(14), 143122 (2007). [CrossRef]
  21. B. Zhu, Y. Feng, J. Zhao, C. Huang, and T. Jiang, “Switchable metamaterial reflector/absorber for different polarized electromagnetic waves,” Appl. Phys. Lett.97(5), 051906 (2010). [CrossRef]

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